The present invention relates to a coherent optical receiver which employs optical heterodyne detection and is for optical communication or optical measurement.
FIG. 5 shows a conventional coherent optical receiver .alpha. comprising a local oscillator light source 1, a light combining/dividing unit 2, optical detectors 3 and 4, and a first-stage electrical amplifier 5. The local oscillator light source 1 is a semiconductor laser-diode or the like, and generates a local oscillator light Lo. The light combining/dividing unit 2 is a beam splitter, an optical fiber coupler or the like, and operates so that a signal light S which is weak and transmitted through an optical fiber or the like and the local oscillator light Lo are combined and divided at a branching ratio of 50:50. The optical detectors 3 and 4 which make a dual balanced receiver 12 are for performing heterodyne detection to make beat signals from the signal light S and the local oscillator light Lo. The electrical amplifier 5 amplifies an output signal O resulting from the synthesis of the beat signals. The weak signal light S and the local oscillator light Lo are combined and divided by the light combining/dividing unit 2, the outputs from which are received by the optical detectors 3 and 4 so that the outputs undergo the heterodyne detection due to the squaring properties of the receivers to generate the beat signals which are electrical signals. The frequency of each of the beat signal is equal to the difference between those of the signal light and the local oscillator light. At the heterodyne detection, the signal light S is changed into an intermediate frequency band. The signal power to noise power ratio (SNR) in the band is generally represented by an equation as follows: ##EQU1## Ro: Photoelectric conversion coefficient ##EQU2## .eta.: Quantum efficiency of optical detector e: Charge of electron
h: Planck's constant PA1 .nu.: Frequency of light PA1 PS: Power of signal light to optical detector PA1 PLo: Power of local oscillator light to optical detector PA1 Id: Dark current in optical detector PA1 F: Noise factor of electrical amplifier PA1 K: Boltsmann's constant PA1 T: Absolute temperature PA1 S: Intermediate-frequency bandwidth PA1 R: Load resistance of optical detector
It is understood that the signal power S shown by the numerator of the equation is proportional to the product of the power PS of the signal light and the power PLo of the local oscillator light. The noise power N shown by the denominator of the equation is the sum of the shot noise due to the local oscillator light Lo, that due to the signal light S, that due to the dark current in the optical detector and the circuit noise of the electric amplifier, which are denoted by the first, second, third and fourth terms of the denominator. Since the signal light S is weak and the dark currents in the optical detectors 3 and 4 are very small, the shot noise due to the signal light S and those due to the dark currents are negligible.
FIG. 6 is a graph having an ordinate axis for the signal power S and the noise power N, and an abscissa axis for the power PLo of the local oscillator light Lo, and showing the relationship between the local oscillator light power and the optical receiver sensitivity of the optical receiver .alpha. in the case where the load resistance thereof is 50.OMEGA. for high-speed transmission and the equation is substituted with general numerical values. It is understood from FIG. 6 that the SNR is restricted by the circuit noise of the electric amplifier when the local oscillator light power PLo is low (about 10 dBm or less), but the ratio is restricted by the shot noise due to the local oscillator light so as to create an ideal heterodyne detection state, when the local oscillator light power is high (about 10 dBm or more). The latter area is a theoretical margin in the heterodyne detection. The margin is a called a state of shot-noise limit.
As described above, when the local oscillator light power PLo is not high, the SNR or optical receiver sensitivity of the conventional coherent optical receiver is restricted not by the shot noise due to the local oscillator light power but mainly by the circuit noise of the electric amplifier so as not to be high. This is a problem. The local oscillator light power PLo needs to be high if the state of shot-noise limit is to be achieved. In other words, the local oscillator light power PLo needs to be about 10 dBm or more if the state of shot-noise limit is to be achieved, as mentioned above. The local oscillator light source 1 is made of a semiconductor laser-diode with which it is difficult to make the local oscillator light power PLo as high as about 10 dBm or more in present technology. If the local oscillator light power PLo were made capable of being rendered as high as about 10 dBm or more, the reliability of the local oscillator light source 1 could deteriorate. Therefore, it is not easy to set the local oscillator light power at such a high level. Although the state of shot-noise limit can be achieved by reducing the circuit noise of the electrical amplifier 5, the input impedance of the amplifier or the load resistance of the optical detector needs to be decreased if signals in a wideband are to be transmitted. However, as understood from the equation, if the load resistance is reduced, the circuit noise increases to lower the SNR or become uneasy to be decreased.
"Balanced dual-detector receiver for optical heterodyne communication at Gbit/s rates, ELECTRONICS LETTERS, 10th Apr. 1988, Vol. 22, No. 8" and "Local-oscillator excess-noise suppression for homodyne and heterodyne detection, OPTICS LETTERS, August 1983 Vol. 8, No 5" show the above-mentioned prior art.